MAZAMA ASH USED FOR PRECISE CHRONOSTRATIGRAPHY OF LAKE SUPERIOR SEDIMENT CORES

Tephrochronology is a stratigraphic method used to correlate and date geologic, climatic and environmental events in the recent geologic past. Its precision is rarely matched by other methods, due to the relatively instantaneous deposition of tephra shards.

This study examined three Lake Superior sediment cores: near Split Rock, Isle Royale, and the Keweenaw Peninsula. We searched for cryptopephra (i.e., tephra layers not apparent by visual inspection alone), derived from the eruption of Mount Mazama, ~7600 B.P., which created Crater Lake in Oregon. Previous studies have identified the Mazama ash in northwestern North America, Newfoundland, and the Greenland Ice Sheet. There have been no previous observations of the Mazama ash in sediments recovered from the Laurentian Great Lakes.

Because of the large distance between Lake Superior and Mount Mazama, no obvious tephra layer associated with this eruption is observed. We estimated tephra layer location based on approximate age determined by paleomagnetic secular variation (PSV) in Lake Superior cores, isolated the tephra shards, and prepared smear slides for each ten centimeters along the core section. These slides were viewed under a petrographic microscope, and tephra shards were identified and counted based on morphology and optical characteristics. The ten centimeter interval with the highest abundance of tephra shards was further sampled every centimeter to more accurately determine tephra layer horizons.

Distinct cryptotephra layers were found in all three cores from Lake Superior. Based on size, estimated age based on PSV, and morphological and optical characteristics, we hypothesize that these tephra shards are derived from the Mount Mazama eruption. This has significant implications for the precise chronostratigraphy of Lake Superior sediment cores. Electron Microprobe analysis is currently planned to test our hypothesized source of the volcanic debris.